Nitrocellulose, which is processed into various types of propellants and explosives, is manufactured from cellulose feed stocks consisting of fine fibers of cotton linters or macerated wood pulp.
In order to produce nitrocellulose, the cellulose feed stocks are treated with nitric acid to replace hydroxyl radicals in the cellulose structure with nitrate radicals. However, water is formed as a reaction product, and since the reaction is reversible, the water must be removed from the reaction site or the nitration reaction will not proceed to the desired degree. The nitration reaction can be written as follows: EQU Cellulose+Nitric acid.revreaction.Nitrocellulose+Water
Water heretofore has been removed from the reaction site by introducing sulfuric acid together with the nitric acid reactant into the reaction mixture. The sulfuric acid takes up the water, thus permitting the nitration of the cellulose to proceed. This process requires a purification and stabilization step to remove all traces of sulfuric acid. This is necessary because any sulfuric acid remaining in the product acts to break down the nitrocellulose resulting in degradation of the product. Heat and fumes may be evolved and explosion or fire may ultimately occur. The resulting product would not be suitable for processing into propellants or explosives with long shelf lives and critical performance and physical characteristics.
The required purification and stabilization of the nitrocellulose formed in the nitric acid-sulfuric acid reaction involve long boiling and washing operations which consume large quantities of heat, water, steam and electricity, requiring a large and costly facility and stringent quality controls. Although the resulting product may be satisfactory, the process is long and costly and large quantities of contaminated water are produced, causing pollution problems.
Efforts have been made to develop a process which eliminates the need for using sulfuric acid, with little success. One approach has been to use high concentrations of nitric acid so the water produced by the reaction is taken up in diluting the concentrated acid. This is not satisfactory because the dilution of the acid occurring near the cellulose surface causes the acid concentration to reach a point where the nitrocellulose dissolves and precipitates in crystalline form, causing the surface to harden. This is known as gelatinization and it makes the nitrocellulose unsuitable for processing into propellants.
If the concentration of the nitric acid is kept below the level where gelatinization takes place, the water produced in the nitration reaction reduces the concentration to the point where the reaction becomes reversible and the nitration reaction cannot proceed to the desired degree.
The above demonstrates that there is a need for a process, either continuous or batch, which will accomplish the nitration of cellulose by reaction of nitric acid and cellulose without the presence of sulfuric acid and which will provide for the economical elimination of water to produce a nitrocellulose which is homogeneous and suitable for use in propellants.